1. Field of the Invention
The present invention relates to a sheet supplying apparatus used with an image forming apparatus such as a copying machine, a printer, a facsimile and the like and an automatic original feeding apparatus.
2. Related Background Art
Among sheet supplying apparatus for supplying a recording sheet or an original (referred to as generically "sheet" hereinafter) in an image forming portion or an image reading portion of an image forming apparatus, there is a sheet supplying apparatus having a sheet stacking tray on which sheets are stacked and which can be lifted and lowered. In such sheet supplying apparatuses having the sheet stacking tray, a fundamental construction of a so-called sheet supply deck for containing a large number of sheets and for supplying the sheet to an image forming portion will be briefly described hereinbelow.
The sheet stacking tray can be lifted and lowered by a drive source such as a motor. After the sheets are stacked on the tray at a lowered position, the tray is lifted upon closing a cover. When a sheet surface detection sensor is contacted with an uppermost sheet to be turned ON, the drive source is stopped to stop the lifting movement of the sheet stacking tray.
The sheets stacked on the sheet stacking tray are successively supplied from an uppermost one by a pick-up roller to an image forming portion. After the sheets stacked on the sheet stacking tray are successively supplied, when the position of the upper surface of the sheet stack is lowered to disengage from the sheet surface detection sensor, the sensor is turned OFF to lift the sheet stacking tray again until the sheet surface detection sensor is turned ON. In this way, the upper surface of the sheet stack on the sheet stacking tray is maintained at a sheet supplying position having a certain level range.
The pick-up roller for supplying the uppermost sheet from the sheet stack on the sheet stacking tray is lifted when a pick-up solenoid is turned ON and is lowered by its own weight to be contacted with the upper surface of the sheet stack on the sheet stacking tray when the pick-up solenoid is turned OFF. The pick-up roller is lowered at a predetermined sheet supplying timing in response to a sheet supply signal regardless of the lifting movement of the sheet stacking tray to be contacted with the uppermost sheet in the sheet stack. When the pick-up roller is rotated, the uppermost sheet is fed out.
However, the above-mentioned conventional sheet supplying apparatus in which the sheet is fed out by the pick-up roller which can be lifted and lowered has the following drawbacks.
After the sheet surface detection sensor is turned OFF due to the reduction in height of the sheet stack on the sheet stacking tray, while the sheet stacking tray is lifted, when the pick-up roller is lowered to feed out the sheet in response to the sheet supply signal, the lowering pick-up roller strikes against the uppermost sheet in the sheet stack rested on the lifting sheet stacking tray to be bounded more greatly than the case where the sheet stacking tray is stopped, thereby arising the following problems.
Firstly, if the rotating pick-up roller strikes against the uppermost sheet in the sheet stack rested on the lifting sheet stacking tray, the surface of the sheet and/or the outer peripheral surface of the pick-up roller is rubbed and/or contaminated. If the peripheral surface of the pick-up roller is rubbed, the service life of the apparatus is worsened. If the sheet is rubbed, when the sheet is a recording sheet, quality of a transferred image is worsened, while when the sheet is an original, the original is greatly damaged.
If the pick-up roller is bounded on the sheet stack, a sheet supplying timing is delayed, so that the sheet does not reach a predetermined position within a predetermined time period. Thus, delay sheet jam frequently occurs. Further, due to the bounding of the pick-up roller, the sheet is skew-fed, that is the sheet is not separated correctly at a separation portion to cause the poor sheet separation or the sheet jam due to the skew-feed of the sheet.
Particularly, in recent image forming apparatuses and automatic original feeding apparatuses, since the sheet is conveyed at higher speed to decrease a sheet interval in the continuous sheet supply, the sheet supplying timing is hastened to frequently generate the lifting movement of the sheet stacking tray and the lowering movement of the pick-up roller simultaneously. Thus, the above problems caused by the bounding of the pick-up roller is apt to arise.